Free radical reactions curing mechanisms

It should be noted that in addition to the use of amines and peroxides there have been other initiating reaetions reported as methods to generate free radicals and cure aerylie adhesives. One example is shown in Fig. 4. In addition to this example, hterature searehes will reveal numerous other mechanisms to initiate cure. As seen in Fig. 4, organic compounds used to initiate the reaction can vary considerably however, they all tend to result in the formation of critically important free radicals. These radicals are eapable of... 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

The curing mechanism of this type of adhesive is a free radical reaction. Thus, the usual steps in such a reaction also take place in these types of adhesives. The adhesive has to contain a free radical initiator, the breakdown of which into free radicals is the initiation step for the reaction. The next step is the propagation step in which the free radical initiator species reacts with the monomer and then the initiator/monomer reaction product (also a free radical) continues the reaction until all of the monomer is consumed as part of the polymer. Next steps are termination reactions in which free radical ends of the polymer find each other and react, terminating the reaction. Otherwise, the propagating free radical can undergo disproportionation, also resulting in termination. 

Hydrosdylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro sdylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

Resin-modified glass—ionomer lining and restorative materials add a multifunctional acidic monomer to the poly(acryhc acid) [9003-01 Hquid component of the system. Once the glass powder and Hquid are mixed, setting can proceed by the acid—glass—ionomer reaction or the added monomer can be polymerized by a free-radical mechanism to rapidly fix the material in place (74,75). The cured material stiH retains the fluoride releasing capabiHties of a glass—ionomer. 

Epoxy acrylates are also commonly used as oligomers in radiation-curing coatings and adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation. These resins are formulated with photoinitiators to cure via uv or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. The uv/EB cured epoxy formulations are discussed in Chap. 14. 

UV Cure Mechanisms. There are two fundamental mechanisms for the uv curing of coatings free radical and cationic. The more common free radical mechanism consists of a chain reaction with four primary steps ... 

There are essentially two different mechanisms that UV curing may occur by — free radical or cationic. Free radical polymerization is the most predominantly used route and will be discussed first. The chain reaction that occurs consists of at least four steps ... 

Previous investigations have shown that polyisobutene and poly(isobutene-co-iso-prene), i. e. butyl rubber, are unable to cross-link in the presence of free radicals, since extensive chain scissions occur and thus low molecular weight products are formed The degradation mechanism proposed by Loan involves, in the case of polyisobutene, the H abstraction from methyl groups followed by chain scission. Apparently, the formation of secondary alkyl radicals, which are believed to be responsible of polyolefin radical curing is prevented for steric reasons by the presence of two adjacent dimethyl substituted carbon atoms and hence j3 scission reactions prevail. 

Initiators are used to initiate the curing reaction at elevated temperatures. Cross-linking or polymerization occurs by a free radical mechanism in which the double bond of the polyester chain reacts with the vinyl monomer that is usually styrene, and this reaction provides a three-dimensional network that converts the viscous resin to a hard thermoset solid. The initiators added decompose at elevated temperatures thus providing free radicals to initiate the cross-linking. Peroxyesters and peroxyketals are the most common classes of peroxides used as initiators. 

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